Atherosclerosis, characterized by clogging, narrowing, and hardening of the body's large arteries and medium-sized blood vessels, can lead to stroke, heart attack, eye problems and kidney problems. It is a leading cause of mortality and morbidity worldwide. Growing evidence suggests that an important factor in assessing the increased risk associated with an atherosclerotic plaque deposit is the physical composition of the plaque. Indeed, the plaque composition is a better indicator of risk than the degree of luminal narrowing as measured by angiography.
Atherosclerosis is characterized by the deposition of plaques containing cholesterol and lipids on the innermost layer of the walls of arteries. Atherosclerosis is currently understood to be a chronic inflammatory disease rather than an inevitable degenerative aging process. The condition usually affects large- and medium-sized arteries. Although such plaque deposits can significantly reduce the blood's flow through an artery, the more serious risk is generally associated with the instigation of an acute clinical event through plaque rupture and thrombosis. In particular, serious damage can occur if an arterial plaque deposit becomes fragile and ruptures, fissures, or ulcerates. Plaque rupture, fissure, or ulcer can cause blood clots to form that block or occlude blood flow and/or break off and travel to other parts of the body. If such blood clots block a blood vessel that feeds the heart, it causes a heart attack. If the blood clot blocks a blood vessel that feeds the brain, it causes a stroke. Similarly, if blood supply to the arms or legs is reduced, it can cause difficulty in walking or light exercise and other collateral damage.
The presence and extent of plaque build up in an individual's arteries can be detected using a variety of techniques that are well known in the field including, for example, magnetic resonance imaging (“MRI”), computed tomography (“CT”), X-ray angiography, and ultrasound. Prior art methods for assessing an individual's risk of a clinically significant event such as a stroke or heart attack related to atherosclerotic deposits in an individual's arteries have primarily been directed to evaluating the effect that the plaque deposit has on the blood flow through the artery.
The risk associated with rupture, fissure, or ulceration of plaque, however, may be present even when the plaque deposit does not significantly reduce the flow of blood in an artery. For example, arteries and other blood vessels will sometimes expand or “remodel” in the region of a significant atherosclerotic plaque deposit such that the lumen area does not decrease sufficiently to significantly reduce blood flow. If the plaque ruptures, it may nevertheless create a blood clot that may travel to a critical area to cause a clinical event. The susceptibility of a plaque deposit to structural failure is difficult to determine.
Numerous studies have shown that MRI can detect differences in plaque composition, but that combined information from images with multiple contrast weightings is critical for distinguishing all plaque components. Desirable combinations of contrast weightings and a set of image characteristics have emerged that can be used to segment plaque into its subcomponents. Manual segmentation using these characteristics has produced quantitative measurements of the relative volumes of necrotic cores, calcification, loose matrix and fibrous tissue that correlate strongly with histological assessments. Efforts have been made to automate the segmentation of plaque components, focusing on the intensity characteristics of the plaque in the clinical images. Although somewhat successful in experiments on specimens imaged ex vivo, these methods have failed in vivo.
There remains a need for a relatively simple method and system for automating the identification and segmentation of the composition of an atherosclerotic plaque deposit in a patient's artery.